An infrared sensor can detect infrared light being radiated from an object such as a human body. With the ability for non-contact detection of the existence or the temperature of an object, it is expected to find applications in a variety of fields of technology. Particularly, an infrared sensor array including a plurality of infrared sensors arranged in a matrix pattern is capable of obtaining a two-dimensional infrared light image, and is expected to find applications in an even wider variety of fields of technology. Favorable candidates for the infrared sensors used in such an infrared sensor array include resistive bolometers and dielectric bolometers detecting a change in the dielectric constant caused by a temperature change through the application of an electric field, because they do not require cooling or a chopper circuit.
Conventional resistive bolometers and dielectric bolometer-type infrared sensors with configurations as follows are known in the art (Patent Documents 1 and 2).
FIG. 17 shows a signal reading circuit of a conventional resistive bolometer-type infrared sensor. As a first switch 154A, a second switch 154B and a third switch 154C are turned ON, the difference between the output of a dummy resistor 152 and the output of a heat-sensitive resistor 151 is output to an output 160 of the signal reading circuit. In order to accurately read the value detected by the heat-sensitive resistor 151, the resistance value of the dummy resistor 152 needs to be constant. However, the resistance value of the dummy resistor 152 varies depending on the temperature of the semiconductor substrate, on which the dummy resistor 152 is formed. Thus, while it is necessary to accurately measure the temperature of the semiconductor substrate in order to correct a variation of the resistance value of the dummy resistor 152, it is not easy to precisely detect the temperature of the semiconductor substrate.
FIG. 18 shows a signal reading circuit of a conventional dielectric bolometer-type infrared sensor. As shown in FIG. 18, a reference capacitor element 201 and an infrared-detecting capacitor element 202 are connected in series with each other via a node 210. The infrared-detecting capacitor element 202 has characteristics such that the capacitance thereof varies depending on the intensity of infrared light incident on the element. The characteristics of the element are set so that the capacitance value of the infrared-detecting capacitor element 202 and that of the reference capacitor element 201 are equal to each other when there is no infrared light incident thereon.
An alternating-current power supply 204 and an alternating-current power supply 205 are connected to the reference capacitor element 201 and the infrared-detecting capacitor element 202 for driving the capacitor elements 201 and 202, respectively, wherein the alternating-current power supply 204 and the alternating-current power supply 205 have the same amplitude and inverted phases.
The node 210 is connected to an output terminal 206 via a transistor 203, and the potential of the node 210 can be taken out to the output terminal 206 by turning ON the transistor 203 via a signal line SSW.
The potential of the node 210 is determined by the capacitances of the reference capacitor 201 and the infrared-detecting capacitor 202 and the voltages (amplitudes) of the alternating-current power supply 204 and the alternating-current power supply 205. Therefore, when infrared light is incident on the infrared-detecting capacitor element 202, whereby the capacitance value of the infrared-detecting capacitor element 202 increases as shown in FIG. 19, there is obtained an output curve as shown by an output curve A in FIG. 19. In FIG. 19, a curve C and a curve D represent output voltages of the alternating-current power supply 204 and the alternating-current power supply 205, respectively.
When there is no infrared light incident on the infrared-detecting capacitor element 202, the capacitance value of the reference capacitor element 201 and that of the infrared-detecting capacitor element 202 are equal to each other, whereby the potential of the node 210 is supposed to be always zero as indicated by B in FIG. 19. In practice, however, there is a difference on the order of 1%, due to the leak resistance component, variations occurring during the formation process, etc., between the capacitance value of the reference capacitor element 201 and that of the infrared-detecting capacitor element 202. Therefore, even if there is no infrared light incident on the sensor, there occurs an offset, being a spurious signal output.
Using the output of an infrared sensor as digital data requires an amplification by a factor of about 100. Then, the offset will also be amplified 100 times, which may saturate the amplifier circuit. It is also possible that the signal becomes hidden behind the substantial offset. Therefore, in order to realize a high-performance infrared sensor, it is necessary to correct and reduce the offset.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-227689
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-365130